In a cased oil or gas well, the hydrocarbon in the formation can be accessed by perforating the casing with a high-energy shape charge or by abrasively cutting holes or slots in the casing with a jetting tool. In the latter application, slurry is pumped down a tubular and through a small jetting nozzle. This abrasive mixture exits the jetting tool at a high velocity, impinges on the casing wall and abrades or cuts holes in the casing. Abrading holes in casing is performed by technologies such as the Abrasijet™ tool introduced by Schlumberger.
Conventional jetting assemblies are lowered on drillpipe. Some drillpipe conveyed jetting assemblies include slip-type mechanisms to limit the vibration of the bottom hole assembly (BHA) in the wellbore, however, these slips are not designed to stop axial movement of the BHA in the wellbore.
Recently, jetting tools have been attached to coiled tubing and this has introduced new challenges. The primary issue facing coiled tubing deployed jetting is depth control. Knowing exactly where the BHA is during a job and maintaining the BHA in a desired location during operations is difficult. The coiled tubing length is susceptible to axial compression and tension forces, internal pressure, flow rate down the tubing or annulus, high temperatures, coiled tubing friction with casing wall, etc. During jet cutting and other wellbore operations, many of the forces mentioned act on the tubing and BHA. The result is that the overall length of the coiled tubing changes and the tool moves during the operation. Movement of the jetting tool during cutting operations results in slots or incomplete cutting of the casing. In a worst-case scenario, the jetting tool can move as much as ten ft (3 m), which can be enough to jet holes into the wrong formation behind the reservoir.
Conventional techniques for maintaining depth control of coiled tubing include devices that monitor how much tubing has been fed into the wellbore, however these techniques do not provide the extent of buckling, stretch, etc. Enhancements to these methods include the step of using forward modeling or knowledge of the tubing properties to predict this buckling, stretch, etc.
Depth control during abrasion cutting has conventionally included the step of using a mechanical casing collar locator (CCL) that activates a hammer to “strike” the coiled tubing each time the CCL crosses a casing collar. The sound of the hammer striking the coil can (sometimes) be picked up by listening to the coil at the surface.
Therefore, there is a desire to provide methods and systems for controlling the depth of a coiled tubing conveyed tool during wellbore operations.